1. Field of the Invention
This invention relates to a supercharger of an internal combustion engine having a Roots compressor or pump. More particularly, it relates to a lubrication system for bearings supporting the rotor shafts of a Roots-type pump.
2. Description of the Related Art
A Roots-type pump of a supercharger for an internal combustion engine comprises two hour-glass-shaped rotors mounted on respective rotor shafts. A pumping operation is effected in the pump housing by rotating these rotors synchronously in opposite directions. A small clearance is maintained between the respective rotors and between the rotors and the inner wall of the housing, to compensate for thermal expansion of the rotors or to cope with foreign matter carried into the housing by intake air. These clearances allow the compressed discharge air to leak from the high pressure area to the low pressure area in the supercharger, with the result that the pressure of air discharged from the high pressure area will impinge on the bearings supporting the respective ends of the rotor shafts, and thus a pressure difference is created at opposite sides of the bearings. Each of the bearings is packed with grease for lubrication, as is well known in the art. However, the sealing or holding property or lubricating capacity of the grease may be reduced by this pressure difference.
To prevent a reduction of the holding capacity of the grease caused by the pressure difference mentioned above, a device has been proposed to provide an air passage for communicating the clearance space in the housing with the suction side of the pump, as disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 59-54785.
Another solution was proposed in which labyrinth is provided between the rotor and the bearings in such a manner that the clearances between the bearings and the labyrinth are connected to the atmosphere to reduce the suction pressure normally acting on the bearings.
In a Roots-type pump of the supercharger of the internal combustion engine, the above-mentioned clearances of the labyrinth between the bearings and the shafts must be necessarily large enough to avoid contact therebetween, and the communicating port to the atmosphere must also be large enough to obtain a reduction of the suction air pressure. In this case, however, in a particular engine driving condition, such as idling in which the flow of suction air is small, the atmospheric pressure may be drawn into the engine through the above-mentioned labyrinth and thus the flow of the suction air will be substantially increased. Accordingly, it will be difficult to reduce the engine speed, even if the throttle valve is almost fully closed to reduce the amount of suction air drawn into the engine.